Process for drying magnesium sulphate



Oct. 22, 1940. L, D, Rica-IARDS I 2,218,551V

PROCESS FOR DRYING MAGNESIUM SULPHATE Filed April 30, 1938 7; Mixer Y @950% 719g@ Wafer' BY@ il/PAE A TTORNEYS.

Patented Oct. 22, 19,40 y

N UNITED STATES PROCESS Fon DRYING MAGNESIUM SULPHATE Le@ D. Richards, Numana, Mich., assigner to The Dow Chemical Company, llidlan'd,` a corporation of Michigan i application April 30,19%, serial Nazosss y L" 7 claims.' (c1. ca -12s) The invention relates to the vdrying `oi hydrated magnesium sulphate, or Epsom salt, `to produceV a hard, dense, granularlproduct. l

The Epsom salt of commerce .is a hydrated 5 crystalline salt containing 7 molecules of water of crystallization, the formula being MgSOaZI-IzO,

For some uses in the arts, however, a partially dehydrated or anhydrous product is desired. 1 However, drying magnesium sulphate heptahydrate byany of the well known methods of drying` a hydrated salt generally produces a product unsatisfactory for commercial use. For example, when MgSOiJlI-IzO crystals are heated rapidly to drive oli the water of crystallization, they melt andY form a fused mass which isthen diflicultto i dehydrate and incapable of yieldinga marketable product. On the other hand, if the heating is ,carried out slowly, as by air drying at a moderate temperature under vacuum toavoid melting the 2,0 crystals, they become porous, soft and extremely light, so that they crumble'and disintegrate into Va line powder or dust, both during the drying and the subsequent handling. Due to its nelydivided form, a substantial loss of material occurs during drying. Such a product is diflicult to handle and, being very light, must be packaged inxbulky containers if it is to be shipped.

It"is, therefore, among theobjects of the invention to provide a method of dehydrating mag- `nesium sulphate heptahydrate which is capable of 'yielding a product `in the form of dense, hard, granular vparticles rather than `a light powdery material. f

I have found that the aforementioned dilcultiesand disadvantages can be overcome by mixing the normal hydrated salt (MgSO4.7HzO) crystals withla relatively hot mass `of either granular or pulverulent partially or completely` dehydrated Epsom salt under conditions such that the .hydrated salt is meltedV and absorbed by or upon the particles of the partially or completely dehydrated material. I'he product obtained is in the form of relatively hard granular shot-like particles substantially free from lumps and dust. The hard ,45 dense partially dehydrated granules so obtained maybe then further dehydrated to a lower -water content, if so desired, without material loss "ofits hard structure.

'The invention, then, consists of the process hereinafter fully (described vand particularly pointed out in the claims, the annexed drawing and the following descripton'setting forth in detail a mode of carrying out the invention;`

The following detailed vdescription of my im- .5 proved lprocess may be readily followed by reference to the accompanying drawing, the single figure Aof which4 illustrates diagrammatically a sequence of steps for carrying out the process. As indicated in the drawing, the process is started by first preparing a quantity of dehydrated pul- 5 verulent Epsom salt containing about-three or'less molecules of water of crystallization.` Such material is conveniently prepared byslowly drying MgSO4-7H2O at a moderate temperature in a conventional .type ,dryert `The initial drying tem- 10 perature should be slightly above room temperature (e. g. 40 C.)` and as the Adrying slowly proceeds the temperature is vgradually raised, the iinal drying temperature depending on the degree of dehydration desired. A final temperature V1li of from 100-.140 C. producesaMgSOicontaining about three molecules of water crystallization, while ahigher temperature of around 180-200 C. gives MgSO4.H2O-. u A still higher temperature above 200.' C. produces anhydrous magnesium 20 sulphate. Any vof these partially or completely dehydratedv pulverulent magnesium sulphates so,

`or otherwise produced may `be suitably used to start the process. l

A Yquantity of dehydrated Epsom salt, such as 25 produced as described above, is heated to a temperature between about 100 and 300 C. ormore and to this `hot mass isadded Epsom salt crystals while the mass is continually stirred, the amount of Epsom salt added` being about 1/5 to 1 part by 30 .weight of that' of the dehydrated Epsom salt. e Such mixing may be rbrought about in any conventional typerlixer. The addition ,is made at such rate that the' temperature of the mixture does not fall substantially below C. or materi- 35 ally exceed 140 C., the preferred temperature range being -120 C. It is necessary that the amount of the heptahydrate crystals added tothe partially or completely dehydrated salt, as well as the amount of the latter and its temperature, be o such that the temperature' of the resulting mixturel is no'tless than about 85 C. or moregthan about C. .If the temperature of the mixture is less than about 85 C.`the heptahydrate crystals do not melt at'rst but form a mechanical 45 mixture whichwhen subsequently heated to a higher temperature to bring about dehydration, produces aproduct which is in the form of a iine powder or dust. On the otherhand, if the temperature` of the mixture materially exceeds 140 50 C., the particlesof dehydrated material are at a temperature so far above the temperature at Which melted MgSOiflHz'Ofboils that the melted MgSOlJlI-IZO does not melt upon and penetrate the `particles of dehydratedmaterial, `but instead 55 the melted MgSO4.7I-I2O is explosively expelled in fine particles from the surface of the dehydrated material due to the high temperature, so that fuither heating of this mixture produces a large amount of powdery material instead of hard shotlike particles. However, I have found that, when they temperature of the partially dehydrated Epsom salt and the amount used relative to that of the Epsom salt to be dried is properly regulated as stated above, such difficulty is entirely avoided. Y,

By heating the partially dehydrated'Epsom salt to the preferred temperature (100 C.-to,120 C.) and then adding the right proportion of the Epsom salt to be dried, the heptahydrate crystals melt quickly, 'and the molten hydrated salt, on being thoroughly intermixed with the hot partially or completely dehydrated salt particles, forms a lm of` molten salt upon the particles of dehydrated material which becomes absorbed by the latter. Thereby a mass of non-coherent granules resembling shot in shape is formed in which the individual shot-like particles 'at the prevailing temperature do not coalesce to form larger aggregates.

It is important as already indicated to employ the proper` proportions of heptahydrate crystals relative to the partially or completely dehydrated Epsom salt. In general, one part by weight of heptahydrate to four parts by weight of the dehydrated material is most satisfactory, although the proportion may be'varie'd from about one to live parts of the dehydrated material to one part of heptahydrate, depending on the temperature of the dehydrated material before mixing and the degree of dehydration of this material. For example, satisfactory results Were obtained by mixing magnesium sulphate heptahydrate with partially dehydrated Epsom salt (87 per cent MgSOi) in a ratio of 400 lbs. per hr. yor" the heptahydrate to- 1500 lbs. per hr. of the hot dehydrated Epsom salt, the temperature .of the latter being 275 C. which was suicient to keep the temperature of the mixture at 85C. Other ratios may be employed by properly controlling the temperature of dehydrated material so asl to produce a mixture having a temperaturebetween about 85 to 140 C. l g- After the granular mixture is prepared as described, it may then be subjected to a heating or drying operation to bring about further dehydration, if desired, without loss of`v the hard shotlike particle form. The heating operation may be carried out in'a conventional dryer, such as a shelf dryer provided with means to stir the mixture during the drying. The mixture is heated to such a temperature as will bring about the desired state of dehydration and is continually stirred during the heating operation. 'I'o produce the usually desired MgSO4-1H2O (85-87 per cent MgSOl), the mixture is heated to 180-200 C. in the dryer, while anhydrous MgSO4 may be made by subjecting the heptahydrate-dehydrate mixture to a temperature of above 200 C. A partially dehydrated, hard, granular Epsom salt, having a Water content corresponding to from 21/2 to 3 molecules of water of crystallization (e. g. 68-'75 per cent MgSOi) can be produced in `the mixer of the rst stage, and if 'a product having this water content 'is desired, further drying is unnecessary if the mixing operation iscarried out at the proper temperature and withthe right proportion of ingredients. v

The dehydrated product obtained from the dryer which consistsof hard, dense granules' of completely or partially dehydrated Epsom salt may then be divided into two portions, one of which, While still hot, is returned to the first step of the process as shown in the figure and mixed with the proper proportion of heptahydrate crystals as previously described. Thereafter, the process may be carried out using la portion of the hard dense granular product as the starting material instead of using or specially preparing pulverulent material as was necessary to start the process. The other portion of the hot dehydrated product may be passed through a cooler to cool it to a suitable packaging temperature, after which it may be packaged or otherwise made ready for shipment or use. v

I am aware that it has been proposed to dehydrate various other hydrated salts by procedures employing the intermixing of the hydrated Ysalt with the dehydrated Salt and then further drying the mixture. However, applying these methods to the drm'ng of Epsom salt does not produce; dehydrated Epsom salt having the desired dense granular structure, such as is produced by the present improved process. In the methodspreviously proposed for drying hydrated salts, such as, for example, Na2SO4.10I-I2O, the hydrated salt is mixed with the dehydrated salt while the latter is cold and then the mixture is heated to bring about dehydration. Such procedure, however, when applied to the drying of MgSO4.7I-I2O produces a fine powder, the density of which is only one half of that of the dense shot-like particles produced by the present method.

Various modifications of the described process may be employed Within the scope of the invention. For example, the dehydration of the heptahydrate may be carried out either as a continuous process, as illustrated, or as a batch process, wherein a quantity of heptahydrate is mixed with hot dehydrated magnesium sulphate and the mixture then dried at a suitable temperature to produce a product of the desired Water content. The process may also be carried out by charging the heptahydrate into a rotary dryer of the usual type containing dehydrated magnesium sulphate at the proper temperature. In sucha process, the heptahydrate maybe charged continuously into the rotary dryer if a suflicient quantity of dehydrated Epsom salt is maintained in the dryer at a temperature such that. after mixing lthe temperature of the mixture does not fall vsubstantially below `85" C. or materially exceed The mechanical procedure for carrying out the process can naturally be varied in other Ways, as will be apparent to those skilled in the art.

Other modes of applying the principle vof my invention may be employed instead of those explained, change being made as regards the method herein disclosed, provided the step or steps stated by any of the following claims or the equivalent of suchstated step or steps be employed.

I therefore particularly point out and distinctly claim as my invention:

1. In a method of drying magnesium sulphateheptahydrate to form a hard dense lgranular product, the step which consists in thoroughly intermixing said heptahydrate'with a quantity of hot dehydrated Epsom salt in an amount such that the temperature of the resulting mixture is between about 85-140 C. f

2. In a method of'dr'ying magnesium sulphateheptahydrate to form a dense granular dehydrated product, the steps which consist in thoroughly intermixing said `heptahydrate with a quantity of hot dehydrated Epsom salt, the tem- Vperature and the quantity of `said dehydrated Epsom salt being so controlled that the temperature of the resulting mixture is between about 85 and '140 C., and subjecting the mixture so formed to a drying operation. i 1 f 3.4 In a method of drying magnesium sulphateheptahydrate to form a dense granular dehydrated product, the steps which consist in thoroughly intermixing one part by `weight of. said heptahydrate with from 1 to 5 parts by weight of hot` dehydrated Epsom salt, the temperature of said dehydrated Epsom salt being such that the temperature of the resulting mixture is between about 85" and '140 C.` and subjecting the mixture so formed to a drying operation.

4. In a method of drying magnesium sulphateheptahydrate to form a dense granular dehydrated product, the steps which consist in thoroughly intermixing one part by weight of said i jecting the mixture so formed to a drying operation `at a temperature between about 1009-140 C. to form a dried product having a water content corresponding to substantially 2,1@ to 3 molecules of Water of crystallization.

5. In a method of drying magnesium sulphateheptahydrate to form a dense granular deny-- drated product, the steps which consist in thoroughly intermixing one part by weight of said heptahydrate with from 1 to 5 parts by weight of hot dehydrated Epsom salt, the temperature of dehydrated Epsom salt being such that the temperature of the resulting mixture is between about 85 and 140 C. and subjecting the mixture so formed to a drying operation, at a temperature between about 180-200 C. to form a dried product havin-g a water content correspondsuch that the temperature of the resulting mix-l ture is between about 85 and 140 C. and subjecting the mixture so formed to a drying operation at `a temperature above 200 C. to form a dried product of substantially anhydrous mag'- nesium sulphate.

7. In'a method of drying magnesium sulphateheptahydrate to form a dense granular dehydrated product, the steps which consist in continuously introducing into a mixer magnesium sulphate-heptahydrate, continuously introducing into said mixer-hot dehydrated Epsom salt, the temperature of said dehydrated Epsom salt being such that the temperature of the resulting mixture is between about 85 and 140 C., continuously withdrawing lsome of the. resulting mix-` LEO D. RICHARDS. 

